1. Field of the Invention
This invention in general is related to mechanical fasteners for components that require remote and rapid separation by means of an explosive charge. In particular, the invention describes a reusable fastener for the explosively-actuated separation of multistage missiles in aerospace applications.
2. Description of the Related Art
Multiple-stage rockets require mechanisms for rigidly holding the stages together until separation becomes necessary during flight. Before separation, the stages must be in electrical connection for control purposes; in addition, an umbilical line between stages is often used to provide access to needed utility systems. For example, a rocket comprising a payload stage attached to a booster stage will normally have an electrical interface between the two stages and an umbilical cryogenic line for temperature control of the stages. The electrical interface is typically achieved by means of electrical connector plates that are held together by a compressive fastener that releases the two plates at the moment of separation of the two stages. The fastener structure also contains a cryogenic line running between the two stages, which needs to be cut at the same time the electrical connector plates are separated.
When separation between the booster and payload stages is required, it must occur rapidly and with a minimum of interference between the two components. Therefore, before the payload stage can be physically separated from the booster stage, the electrical interface between the two must be broken and the cryogenic line severed, so that the vehicle and the payload can become totally independent upon release of the fastening device between the two stages. Typically, separation of the electrical connector plates and severance of the umbilical line must be accomplished within a 250 millisecond time window.
Several devices have been developed in the past for fastening components that require rapid separation under difficult operating conditions. For example, U.S. Pat. No. 3,080,815 to Simshauser (1963) discloses a separation device that utilizes an electrical squib to cause the explosive separation of two press-fitted parts attached to adjacent stages of a rocket. The device is designed to function with minimum perturbation to the separated objects, so that a booster stage may be jettisoned from its payload without interfering with the trajectory of the rocket.
U.S. Pat. No. 3,119,298 to Brown (1964) describes another explosively-separable fastener that comprises a piston/cylinder combination of components mounted in slideable connection and held together by a shear pin. An electrical squib in the head of the cylinder is provided for causing the pin to shear and allow the separation of the two components on explosion.
In U.S. Pat. No. 3,185,090 (1965), Weber discloses a dual-charge explosive separation system for multistage rockets. The device is intended to prevent failures resulting from malfunction of single-charge systems.
U.S. Pat. No. 3,393,605 to Parnell (1968) describes a high-pressure explosive actuator for deep-water applications, such as required for oceanography exploration equipment. The device comprises a piston/cylinder combination wherein the travel of the piston is dampened by a spring that limits the effect of the actuating explosive charge.
In U.S. Pat. No. 3,420,470 (1969), Meyer discloses a band-shaped fastener for a satellite separation system. This device is designed to permit the separation of several satellites connected together by a spring-loaded mechanism for launching. Upon release of the tension in the retaining band fastening pairs of satellites, the springs are released and cause the units to separate.
Finally, U.S. Pat. No. 4,062,112 to Lake (1977) discloses a wire cutting device actuated by an explosive charge. A cylindrical barrel contains a slideable piston and an electric squib at one end, and a transverse opening designed to accept a wire inserted therethrough at the other end. When the charge is exploded, the piston is forced through the wire caught in the opening and severs it. The device can then be reset and recharged for further use.
The present invention resulted from an effort to design a light-weight compact, reusable explosively-separable fastener that also comprises a cutter that can be utilized to sever an umbilical cord running between two stages of a rocket. The devices currently used in the industry and described in the prior art are large, cumbersome and heavier than desirable for optimal spacecraft application. Therefore, there is still a need for an improved explosively-actuated fastener that is operable within the stringent specifications of current spacecraft technology.